This invention relates to a method of making selftensioning rings, particularly piston rings, wherein out-of-round blanks, preferably assembled in a stack, are first machined at the inner and outer circumferential faces, and thereafter a segment is removed from the rings to obtain a ring gap.
The invention also relates to a machine for performing the method outlined above. For controlling the radial feed motions of the turning (lathe) tools as a function of the desired circumferential contour, the machine has cams which rotate synchronously with the workpiece holding spindle as well as a linkage mechanism operatively connected with the cams.
It is generally known, for purposes of machine utilization, to work simultaneously on the internal and external circumferences of piston ring blanks with only one machine and to subsequently sever the piston rings in a severing station and thereafter submit them to a final machining while they are clamped such as to present a circular configuration. For the circumferential machining lathes are used which have a tool carrier whose radial feed motion is controlled by cams that rotate synchronously with the work spindle. For an accurate shaping these machines must operate in an extremely accurate manner. Such a manufacturing method further presupposes correct copying cams and a high degree of uniformity and a stress-free condition of the ring blanks to be machined. These conditions cannot always be met and therefore a subsequent machining of the cut-open and circularly clamped piston rings is necessary, that is, the rings are first turned to a diameter which is slightly larger than the desired diameter and during subsequent machining they are turned again and shaped circularly to the desired diameter.
It is further known, as disclosed in U.S. Pat. No. 4,366,543, to determine, by means of a scanning device, the actual out-of-round contour of a piston ring after machining, for the purpose of comparing such contour with a desired contour to effect correction in the out-of-round machining.
It is of importance that after cutting open the piston rings and during circular clamping thereof a parallel gap is obtained in the control ring. For this purpose the piston ring has to be cut open at a defined angle. The extent of cut is obtained from the piston ring contour of the out-of-round piston ring. In conventional cutting machines the setting of the cutting angle is determined empirically. Such a process largely depends from the skill of the operating personnel and requires a plurality of cutting tests. As dimensional criterion the piston ring gap in the control ring as well as the small and the large piston ring diameters are used. Because each diametral difference results in a gap width with a .pi.-factor, the cut can be effected only with a predetermined undersized gap width. Such a gap width tolerance has to be maintained at a relatively large value in order to avoid a continuous subsequent adjustment at the cutting machine. The result is that a further work step is necessary to obtain the desired gap width.